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Full text of "Handbook Of Chemical Engineering - I"

PYROMETRY                                          419
of thermometer is lower than that obtained with mercury-in-glass thermometers. As with industrial thermometers, the conditions under which the instruments are originally calibrated must agree with those of use if errors are to be avoided. The greatest error is introduced, perhaps, when such instruments are calibrated in a liquid, and are used to measure the temperature of a gas.
THERMOELECTRIC PYROMETRY
Thermocouples.—Seebeck discovered in 1821 that if in a closed circuit of two metals the two junctions of the metals are at different temperatures, an electric current will flow in the circuit. For example, if the ends of an iron and of a copper wire are fused together and one of the junctions is heated (within certain temperature limits) positive current will flow in the direction copper to iron at the hot junction or iron to copper at the cold junction. Two causes contribute to the production of the electromotive force which actuates this current: An electromotive force exists between two different metals placed in contact, the magnitude of which depends upon the temperature and upon the metals used; also if a wire of homogeneous material is heated at one end, an electromotive force is developed between the hot and cold ends of the wire, the magnitude of which depends upon the metal and upon the differences in temperature of the ends. These two electromotive forces are known as the Peltier electromotive force and the Thomson electromotive force respectively. The total electromotive force acting in the copper-iron circuit described above is the sum of the Peltier electromotive force at each junction and the Thomson electromotive force over each wire, consideration being given, of course, to the algebraic signs of these four electromotive forces. The total electromotive force acting in a circuit of two dissimilar metals thus depends upon the temperatures of the two junctions. If the temperature of one junction is fixed at that of the room or of melting ice, etc., the temperature of the other junction can be determined by measuring the electromotive force developed in the circuit.
This is the basic principle of thermoelectric pyrometry. The electromotive forces developed by thermocouples are small, usually a few thousandths of a volt. To measure such small electromotive forces special types of sensitive voltmeters (millivolt-meters) or indicators are required. For any particular type of couple these instruments may be graduated to read temperature directly instead of electromotive force.
A simple thermoelectric pyrometer consists of three parts:
(a) The thermocouple of two different metals or alloys having a fused junction (the hot junction) which is inserted into the furnace and the cold junctions which protrude from the furnace and are maintained at some fixed temperature such as that of the room or of melting ice;
(6) Two lead wires, usually of copper, running from the cold junctions of the thermocouple to the indicator;
(c) The indicator which may be a millivoltmeter, a potentiometer or a special type of instrument embodying both of these principles, and may be graduated to read either e.m.f., temperature or both.
Metals Used for Thermocouples.—Although any two dissimilar metals might be employed for a thermocouple, certain combinations are unsatisfactory because of the very small electromotive forces which can be developed and because of the fact that with some combinations the electromotive force may